I'd set aside a gallon and add just a little unflavored loose pipe tobacco. Sample every few days for a few weeks, then add more once it stabilizes.
I imagine too much won't be good but a little might add a nice raisin/spice/leather. My experiences would suggest more dirt, earth and an unpleasant sharpness.
Also, don't be afraid to make a little tea. I learned a lot about oaking making hot water teas then cold diluted vodka teas left for longer periods of time.